The costs associated with storing gaseous hydrogen and transporting it to end users are often far greater than the cost of producing the hydrogen itself. Hydrogen's high storage and transportation costs are attributed to the expense of using high-pressure tanks for containing the gas. Instead, hydrogen can be combined with cheap and abundant atmospheric nitrogen to make ammonia, which is then distributed. Ammonia is storable indefinitely as a liquid at a pressure of about 10 bars at 300 Kelvin.
Systems that make ammonia from nuclear energy are known to the art. The sustained manufacture of very large quantities of ammonia, from nuclear energy, does not appear to be a scientific issue. Ammonia, as made from coal, natural gas, or nuclear energy, may be cheaper on a lower heating value energy basis, than are other liquid fuels as made from these or from petroleum. An autothermal ammonia cracker can convert the ammonia into a hydrogen-containing gas mixture, on demand. The hydrogen is then directed to any of the various applications that use a fuel, or that use hydrogen for any purpose.
U.S. Pat. No. 2,578,193 discloses a heat exchanged, electrically powered, catalytic ammonia decomposer, which operates at about 650° C., and is said to be operable “. . . by unattended small children to produce a gas for the purpose of filling balloons.” U.S. Pat. No. 3,598,538 discloses a heat exchanged, electrically powered ammonia cracker, which can be operated at temperatures approaching 1649° C., but more typically heats the ammonia to 927° C.
The ammonia combustors disclosed in U.S. Pat. Nos. 5,904,910 and 6,488,905 can decompose ammonia non-catalytically by combustion of some of the ammonia with a combination of purified oxygen and air. However, neither of these patents discloses a provision for heat exchanging the combustor inputs and outputs, immediately before and after combustion and decomposition of the ammonia in the combustor. These combustors do not preheat the ammonia and oxygen. Preheating, by heat exchange of inputs and outputs, is necessary for an efficient autothermal recovery of hydrogen from ammonia. Preheating also enables ignition and combustion of ammonia with air as the sole oxidizer.
U.S. Patent Application Publication No. 2005/0037244 discloses a heat exchanged, autothermal ammonia cracker that preheats, combusts, and decomposes a single reactant mixture containing ammonia and air at temperatures up to 1200° C. However, no description is given for a counterflow heat exchanger that separately preheats the ammonia and air and before combustion. Separate preheating prevents ignition of the ammonia and its decomposition products during the preheating step, which confers particular advantages later described herein.
U.S. Pat. No. 7,794,579 further discloses that the ammonia cracker from U.S. Patent Application Publication No. 2005/0037244 may be operated within a temperature range of 200-2000° C. The ammonia cracker appears to operate in the same manner at 200° C. as it does at 2000° C. The specified temperature range was simply broadened beyond 1200° C. without disclosing additional features within the broadened temperature range. No description is given for rapid decomposition of non-burned ammonia molecules that do not contact a catalyst, for any temperature or for any combination of temperature and pressure.
Based on the foregoing, there is a need for a heat exchanged, autothermal ammonia cracker that separately preheats ammonia and an oxygen-containing gas mixture and then combusts them to form a hydrogen-containing gas mixture. The ammonia may be decomposed at combinations of gas pressure and gas temperature that permit rapid and non-catalyzed decomposition of the ammonia.